3.50: Saturation

You could add all the sugar in this scoop to the water in the measuring container, and all the sugar would dissolve, forming a sugar-water solution. A solution is a mixture of two or more substances in which the substances are mixed evenly throughout. The substance that dissolves in a solution is the solute. In this case the solute is sugar. The substance that does the dissolving—in this case, the water—is the solvent. Sugar is one of the most soluble solutes in water. This means that more of it can dissolve in water than is true of many other solutes. However, there is a limit on how much of any solute—even sugar—will dissolve in water or other given solvent.

The maximum amount of sugar that will dissolve in a liter of 20 °C water is 2000 grams. A sugar-water solution that contains 1 liter of water and 2000 grams of sugar is said to be saturated. A
saturated solution
is a solution that contains as much solute as can dissolve in a given solvent at a given temperature. If you add more than 2000 grams of sugar to a liter of 20 °C water, the extra sugar won’t dissolve. On the other hand, a solution containing less than 2000 g of sugar in 1 liter of 20 °C water can hold more sugar. A solution that contains less solute than can dissolve at a given temperature is called an
unsaturated solution
. You can learn more about saturated and unsaturated solutions by watching the video at this URL:
http://www.youtube.com/watch?v=IKimraU21ws
.

Saturation Depends on the Solute

The
Figure
below
shows the amounts of several different solutes that will dissolve in a liter of water at 20 °C. As you can see from the graph, solutes vary greatly in how soluble they are in water. For example, you can dissolve almost 20 times as much sugar as baking soda in the same amount of water at this temperature.

Q:
Assume that a solution contains 150 grams of Epsom salt in 1 liter of water at 20 °C. Is the solution saturated or unsaturated?

A:
A saturated solution of Epsom salt in 1 liter of 20 °C water would contain 250 grams of Epsom salt. Therefore, this solution is unsaturated. It can hold another 100 grams of Epsom salt.

Q:
What do you think would happen if you added more than 250 grams of Epsom salt to a liter of 20 °C water?

A:
Any Epsom salt over 250 grams would not dissolve in the solution.

Summary

A saturated solution is a solution that contains as much solute as can dissolve in a given solvent at a given temperature. An unsaturated solution is a solution that contains less solute than can dissolve in a given solvent at a given temperature.

Solutes differ in their solubility in a given solvent. Therefore, a saturated solution of one solute might contain more or less solute than a saturated solution of a different solute.